This invention relates to a magnetic recording medium for use with a magnetic disk apparatus for carrying out information recording and reproducing operations.
In order to achieve an increase in recording density of a HDD (hard disk drive), a decrease in medium noise is indispensable. In the past, the decrease in medium noise is achieved by improving a film structure or a film material of a magnetic recording medium in order to reduce a product Mrxc2x7t of the magnetic disk, with the help of uninterrupted improvement in output characteristics of a magnetic head. The product Mrxc2x7t is a product of residual magnetization (Mr) of a magnetic layer of the magnetic disk and a film thickness (t) of the magnetic layer of the magnetic disk.
The reduction in Mrxc2x7t is extremely effective in improving R/W (read/write) characteristics but simultaneously causes a problem of a thermal decay characteristic. The decrease in Mrxc2x7t, i.e., the decrease in film thickness of a magnetic layer brings about miniaturization of the grain size of the magnetic layer, resulting in reduction of the medium noise. However, miniaturized crystal grains no longer have a coercive force (Hc) sufficient to hold recorded magnetization as a recorded signal. This results in a phenomenon that the recorded signal is attenuated. This phenomenon is called thermal decay.
In order to prevent the phenomenon (thermal decay) that the recording signal is attenuated, various film structures have been proposed. Attention is recently attracted to one of the film structures which is called an AFC (Anti-Ferro-Coupled-film) structure (see Japanese Unexamined Patent Publication No. 56923/2001 (JP 2001-56293 A)).
A magnetic recording medium disclosed in Japanese Unexamined Patent Publication No. 56923/2001 has a multilayer structure in which a magnetic layer is divided by a nonmagnetic separation layer (Ru, Rh, Ir, or the like) into upper and lower magnetic layers. Specifically, the magnetic layer is divided by the nonmagnetic separation layer into a plurality of magnetic layers having magnetizing directions parallel to one another. Thus, the thermal decay characteristic is improved.
However, a film using the AFC structure is increased in magnetic layer thickness in total due to its structure although the thermal decay characteristic is excellent. The increase in magnetic layer thickness results in a decrease in coercive force squareness ratio (S*). In addition, the pulse width (PW) and the overwrite characteristic are deteriorated. The increase in magnetic layer thickness also results in an increase in grain size of the magnetic layer so that the medium noise (S/N ratio) is deteriorated. Thus, such recording/reproducing characteristics do not fully satisfy recent demands in an increase in recording density.
It is therefore an object of this invention to provide a magnetic recording medium which is excellent in thermal decay characteristic and in recording/reproducing characteristics such as a coercive force squareness ratio (S*), a pulse width, an overwrite characteristic, and medium noise (S/N ratio).
Magnetic recording media according to this invention are as follows.
1) A magnetic recording medium comprising a base body, a first magnetic layer formed on the base body, a second magnetic layer, and a spacer layer formed between the first and the second magnetic layers, each of the first and the second magnetic layers being of a ferromagnetic material, the spacer layer being for inducing antiferromagnetic exchange interaction between the first and the second magnetic layers, the first magnetic layer being for controlling the antiferromagnetic exchange interaction, the second magnetic layer comprising a primary layer and a secondary layer located nearer to the base body than the primary layer, the primary layer having a primary anisotropic magnetic field, the secondary layer having a secondary anisotropic magnetic field which is smaller than the primary anisotropic magnetic field.
2) A magnetic recording medium as mentioned in the paragraph 1), wherein the secondary layer has a thickness smaller than that of the primary layer.
3) A magnetic recording medium as mentioned in the paragraph 1), wherein the secondary layer has a saturated magnetic flux density smaller than that of the primary layer.
4) A magnetic recording medium as mentioned in the paragraph 1), wherein the spacer layer has a surface roughness Rmax of 6 nm or less and another surface roughness Ra of 0.6 nm or less, where Rmax is defined as a maximum height representative of a difference between a highest point and a lowest point and where Ra is representative of a center-line-mean roughness.
5) A magnetic recording medium as mentioned in the paragraph 1), wherein the spacer layer is made of a material comprising Ru. 6) A magnetic recording medium as mentioned in the paragraph 1), wherein the base body comprises a substrate and an underlying layer formed between the substrate and the first magnetic layer. 7) A magnetic recording medium as mentioned in the paragraph 6), wherein the underlying layer comprises an intermediate layer having an hcp (hexagonal close-packed) structure.
8) A magnetic recording medium as mentioned in the paragraph 7), wherein the intermediate layer is formed so that crystal matching or alignment with the primary magnetic layer is improved away from the substrate towards the primary magnetic layer.
9) A magnetic recording medium as mentioned in the paragraph 7), wherein the intermediate layer comprises a plurality of layers.
10) A magnetic recording medium as mentioned in the paragraph 9), wherein one layer of the plurality of layers of the intermediate layer, that is nearest to the first magnetic layer, is made of a ferromagnetic material.
11) A magnetic recording medium as mentioned in the paragraph 6), wherein the substrate is a glass substrate.
12) A magnetic recording medium as mentioned in the paragraph 6), wherein the base body further comprises a precoat layer formed between the substrate and the underlying layer for controlling crystal grains of the first and the second magnetic layers.
13) A magnetic recording medium as mentioned in the paragraph 12), wherein the precoat layer is made of an alloy comprising Cr and Ta.
As described above, the second magnetic layer comprises a plurality of layers including the primary and the secondary layers. In this case, as compared with the case where the second magnetic layer comprises a single layer, a thermal decay characteristic is improved and, simultaneously, a coercive force squareness ratio (S*) and a pulse width (PW) are improved.
As described above, the secondary layer located nearer to the base body than the primary layer has a thickness smaller than that of the primary layer. With this structure, the primary layer relatively thick and located farther from the base body mainly has magnetic recording/reproducing functions while the secondary layer relatively thin and located nearer to the base body has a function of preventing the disturbance in crystal orientation in case where the primary layer is directly formed on the spacer layer.
Specifically, the primary layer having a thickness suitable for magnetic recording/reproducing operations and the spacer layer are not always have lattice constants approximate to each other. Therefore, by providing an additional layer (namely, the secondary layer) having a lattice constant approximate to those of the spacer layer and the primary layer suitable for the magnetic recording/reproducing operations, the difference in lattice constant between the above-mentioned layers can be reduced. The additional layer (the secondary layer) is mainly intended to approximate (match) the lattice constants. Therefore, the additional layer (the secondary layer) is preferably thin. By matching the lattice constants, the disturbance in crystal orientation is suppressed as compared with the case where the primary layer is directly formed on the spacer layer. As a result, the coercive force squareness ratio (S*) and the pulse width (PW) can be improved.
In this case, however, if the secondary layer having an excessively large anisotropic magnetic field is selected as the additional thin layer in order to match the lattice constants, the antiferromagnetic exchange interaction between the first magnetic layer made of a ferromagnetic material controlling the antiferromagnetic exchange interaction and the second magnetic layer may be impeded so that the thermal decay characteristic is deteriorated. In order to avoid such disadvantage, the anisotropic magnetic field of the additional thin layer (namely, the secondary layer) nearer to the substrate must be smaller than that of the thick layer (namely, the primary layer) adjacent to the additional thin layer. Specifically, the crystal orientation is improved and, simultaneously, the antiferromagnetic exchange interaction between the first magnetic layer and the second magnetic layer is not easily affected by the anisotropic magnetic field of the additional thin layer (namely, the secondary layer). Thus, the thermal decay characteristic is improved while the coercive force squareness ratio (S*) and the pulse width (PW) are improved.
Herein, the anisotropic magnetic field of the additional thin layer or the secondary layer (which will be referred to as a lower magnetic layer) or the thick layer or the primary layer (which will be referred to as an upper magnetic layer) can be adjusted by controlling the content of Pt contained in the lower magnetic layer or the upper magnetic layer. A smaller anisotropic magnetic field of the lower magnetic layer than that of the upper magnetic layer can be achieved if the content of Pt in the lower magnetic layer is smaller than that in the upper magnetic layer. Specifically, the content of Pt contained in the lower or the upper magnetic layer is adjusted within a range between 5 and 14 at % (namely, atomic percentages).
By arranging the lower magnetic layer of a small thickness between the spacer layer and the upper magnetic layer, it is possible to prevent the disturbance in crystal orientation in case where the upper magnetic layer is directly formed on the spacer layer. Presumably, this is because the matching in lattice constant between the lower and the upper magnetic layers is improved as described above. As a consequence, the coercive force squareness ratio (S*) and the pulse width (PW) are improved. On the other hand, however, the medium noise (S/N ratio) and the thermal decay characteristic may sometimes be degraded. One of factors causing the degradation in medium noise (S/N ratio) is as follows. The lower magnetic layer mismatching in lattice constant from the spacer layer contains the disturbance in crystal orientation. Therefore, an increase in thickness of the lower magnetic layer causes an increase in noise. Taking the above into consideration, the lower magnetic layer is desired to be as thin as possible, keeping the effect of improving the orientation of the upper magnetic layer.
According to the studies of the present inventors, it has been found out that a smaller saturated magnetic flux density of the lower magnetic layer than that of the upper magnetic layer serves to reduce a noise source of the lower magnetic layer to thereby improve the medium noise (S/N ratio). As a consequence, the coercive force squareness ratio (S*) is improved while the pulse width (PW), the overwrite characteristic, the medium noise (S/N ratio), and the thermal decay characteristic can be improved.
It is noted here that the saturated magnetic flux density of the lower or the upper magnetic layer can be adjusted, for example, by controlling the content of Cr contained in the lower or the upper magnetic layer. A smaller saturated magnetic flux density of the lower magnetic layer than that of the upper magnetic layer can be achieved if the content of Cr contained in the lower magnetic layer is greater than that in the upper magnetic layer. Specifically, the content of Cr contained in the lower or the upper magnetic layer is adjusted within a range between 14 and 24 at %.
In this case, the lower magnetic layer has a thickness within a range between 5 and 80 angstroms, which is preferable in preventing serious deterioration of the S/N ratio. The thickness of the upper magnetic layer is appropriately adjusted in correspondence to a desired value of Mrxc2x7t. The second magnetic layer may comprise two layers, three layers, four or more layers. In view of the production cost, two or three layers are preferable.
As a material of the lower magnetic layer, use may be made of a Co-based alloy such as CoCrPtTa, CoCrTa, CoCrPt, CoPt, CoPtTa, CoCrPtTaB, CoCrPtB, and CoCr (Cr less than 22 at %).
Particularly, use of CoCrPtTa as the lower magnetic layer is preferable because the S/N ratio is improved.
For miniaturization of crystal grains in order to further reduce the medium noise, an element or elements such as O, N, C, H, and H2O may be added to the lower magnetic layer. These elements can be added to the lower magnetic layer using various methods. For example, use may be made of a method of depositing the lower magnetic layer by sputtering in an inactive gas atmosphere using a target containing these elements, a method of depositing the lower magnetic layer by reactive sputtering in a mixed gas atmosphere obtained by mixing O2, N2, NO, NO2, or CH4 and an inactive gas, and so on. In this case, it is preferable that these elements are not excessively added. This is because the addition of these elements as a gas decreases the magnetization of the lower magnetic layer so that the lower magnetic layer does not simultaneously serve as both an orientation control layer and a magnetic layer. The concentration of the gas to be added preferably falls within a range between 0.1 and 2%, more preferably between 0.25 and 1%.
As a material of the upper magnetic layer, use may be made of a Co-based alloy such as CoCrPtB, CoCrPtTaB, CoCrPtTa, CoCrPt, CoCrTa, and CoCr (Cr less than 22 at %). Particularly, CoCrPtB containing Co, Pt, and B is preferable because a high coercive force (Hc) and a high S/N characteristic are achieved.
As a material of the first magnetic layer, use may be made of a Co-based alloy such as CoCr (Cr less than 22at %), CoCrPt, CoCrPtB, CoCrPtTa, CoCrPtTaB, CoCrTa, and CoCrRu. The thickness of the first magnetic layer is appropriately adjusted in correspondence to a desired level of the thermal decay characteristic. Specifically, the thickness of the first magnetic layer falls within a range between 5 and 80 angstroms.
Among the above-mentioned materials of the first magnetic layer, use of CoCr is preferable because the thermal decay characteristic can be improved.
In this invention, the spacer layer preferably has a surface roughness Rmax of 6 nm or less and another surface roughness Ra of 0.6 nm or less. The surface roughnesses referred to herein are specified by Japan Industrial Standard JISB0601 in the manner which will later be described. The action of the spacer layer inducing the antiferromagnetic exchange interaction greatly depends upon the thickness of the spacer layer. It has been found out that, if the surface roughness of the spacer layer is great, variation arises in the antiferromagnetic exchange interaction between the first and the second magnetic layers. In this event, the antiferromagnetic exchange interaction within a plane of the magnetic recording medium exhibits a distribution so that in-plane distribution of the thermal decay is caused. Specifically, it has been found out that, if the spacer layer has a surface roughness given by Rmax of 6 nm or less and Ra of 0.6 nm or less, the above-mentioned variation (distribution) is suppressed.
In order to obtain a predetermined surface roughness of the spacer layer, use is preferably made of a substrate which is mirror-polished to have the predetermined surface roughness or less. Thus, the surface roughness of the spacer layer can easily be adjusted to the predetermined level.
In this invention, it is preferable to use the substrate mirror-polished into Rmax of 6 nm or less and Ra of 0.6 nm or less.
As a material of the spacer layer, use may be made of Ru, Rh, Ir, and an alloy thereof, such as CoRu and NiRu. The thickness of the spacer layer is appropriately adjusted within a range such that the antiferromagnetic exchange interaction is obtained. Specifically, the thickness falls within a range between 4 and 10 angstroms, preferably between 7 and 9 angstroms. Particularly, use of Ru as a material of the spacer layer is preferable because antiferromagnetic exchange interaction is great.
Generally, the magnetic recording medium has a structure in which the underlying layer is formed between the substrate and the first magnetic layer. As the underlying layer, a plurality of layers including an intermediate layer having an hcp (hexagonal close-packed) structure, a lower layer having a bcc (body-centered cubic) structure, a seed layer, and so on may be formed in this order from the side of the first magnetic layer towards the substrate. Alternatively, the underlying layer may be a single layer selected from these layers.
The intermediate layer has an hcp (hexagonal close-packed) structure and is intended to adjust the crystal orientation of the magnetic layer having the hcp structure. For example, the intermediate layer may be made of a material such as CoCr, CoCrB, CoCrPt, CoCrPtTa, and CoCrTa. The intermediate layer may be made of a nonmagnetic or a ferromagnetic material. The intermediate layer may comprise a plurality of layers. The intermediate layer is arranged so that crystal matching with the upper magnetic layer is improved away from the substrate towards the upper magnetic layer (for example, the content of Pt is increased away from the substrate towards the upper magnetic layer). In case where the intermediate layer is used, a film arrangement having an AFC structure may be represented by CoCr/CoCrPtTa/Ru/CoCrPtTa/CoCrPtB, CoCr/CoCrPtTa/CoCrPtTa/Ru/CoCrPtTa/CoCrPtB, and the like. The lower layer has a bcc structure and is mainly intended to improve a magnetostatic characteristic. For example, the lower layer may be made of a material such as Cr and a Cr alloy (for example, CrMo, CrV, CrW, and CrTi). The seed layer is intended to control the grain size of a layer formed thereon. For example, the seed layer may be made of a material such as NiAl, AlCo, CrTi, CrNi, and AlRu.
In case where the intermediate layer is formed, the intermediate layer preferably has a thickness between 5 and 50 angstroms. The thickness greater than 50 angstroms is not preferable because magnetic grains in the magnetic layer are increased in size so that the S/N ratio is decreased. The thickness smaller than 5 angstroms is not preferable because the function of adjusting the crystal orientation of the magnetic layer is insufficient.
In this case, CoCr or CoCrPtTa is advantageously used as the intermediate layer because the crystal matching with the magnetic layer is excellent. In order to further improve the crystal matching, the content of Cr contained in the intermediate layer is smaller than 22 at %. Thus, the above-mentioned function of the intermediate layer (adjusting the crystal orientation of the magnetic layer) is advantageously exhibited.
The material of the substrate is not specifically be restricted. Use may be made of aluminum, glass, glass ceramics, ceramics, silicon, carbon, titanium, and so on. In view of the smoothness of the surface of the substrate, the flatness, the mechanical strength, and the chemical durability, the glass substrate is preferable. Among various glass substrates, an amorphous glass is particularly preferable in controlling the crystals of the film formed on the substrate. As the amorphous glass, use may be made of aluminosilicate glass, borosilicate glass, soda lime glass, or the like. In case where the glass substrate is used, it is presumed that, if microscopically seen, the initial growth of the crystal grains exhibits distribution because the glass contains various components. Therefore, it is desired to include, as the underlying layer, at least a precoat layer controlling the crystal grains of the magnetic layer. Herein, xe2x80x9ccontrolling the crystal grainsxe2x80x9d is addressed to the crystal grain size, the variation (variance) of the crystal grain size, and so on.
Use of an alloy containing Cr and Ta as the precoat layer is preferable because the characteristics such as output (LF: low frequency), the pulse width (PW), and the medium noise (S/N ratio) are remarkably excellent. As the alloy containing Cr and Ta, use may be made of CrTa and CrTaX (X: Ti, O2). The alloy containing Cr and Ta is preferable because an amorphous structure or a substantially amorphous structure promotes the uniformity of initial growth of the crystal grains. In this case, the content of Ta preferably falls within a range between 30 and 80 at %.